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Design and development of heritage mortar for restoration
Author(s)
Loke, Maphole Emelly
Date Issued
2023
Type
Thesis
Publisher
Cape Peninsula University of Technology
Abstract
Historical buildings are a narrative of the countries’ written and unwritten history. Their long-term
existence ensures generational knowledge of past events, milestones, construction
developments and evolution in materials, designs, concepts, and practices throughout the
centuries. It is indisputable that heritage buildings’ survival against human behaviour, natural
disasters, and environmental and atmospheric attacks through the years has proven the use of
durable materials for their construction. Inevitable decay due to long-term exposure to
deterioration factors is a common problem for these monuments. This has resulted in a need to
execute restoration works to reinstate the heritage structures to their original appearance,
physical state, and strength for extended survival.
Several restoration projects in the past and recent times have been performed to reinstate the
missing elements on heritage structures, masonry mortars in particular. Unfortunately, many of
these restoration interventions on these structures have been erroneous and led to poor results
from a restoration standpoint, especially regarding aesthetic criteria. The most common mistake
in restoration work is the use of incompatible restoration materials due to a misunderstanding of
the materials used on these historic wonders. This has resulted in a waste of resources due to
consequential repeated restoration projects and the loss of original structural concepts in terms
of appearance and integrity. These common mistakes can potentially threaten the historical
significance of heritage structures. The problem extends further to designing and producing
mortars that are compatible and their assessment for compatibility and durability.
To overcome the problems associated with material incompatibility, researchers and heritage
restorers have discovered the applicability of analysing original mortars for their chemical,
mineralogical, physical, and mechanical properties preceding the execution of restoration works
(ICOMOS, Venice Charter, 1964). The experimental analysis of these mortars is believed to offer
promising results for the long-term survival of restored historical masonries. To align the current
study with the literature regarding sustainable and compatible restoration of historical mortars,
two phases were examined. The initial phase of this study comprised an experimental analysis of
the samples extracted from the masonry joints, the floor, plaster, and renders of the ancient
colonial edifice in Cape Town, South Africa, Castle of Good Hope built in 1666. The building
precincts on Robben Island were constructed between 1700s and 1800s. A total of nine
representative samples were carefully extracted from the Castle of Good Hope, twelve from the
Pre-primary school building and three samples from the Maximum-security prison on Robben
Island for analysis. The sample size was decided based on material availability, considering the
restriction associated with causing as little destruction as possible on historical structures. Phase 1 of this doctoral thesis involved the determination of the aesthetic properties of the
collected mortars using spectrophotometry, the chemical composition through X-ray fluorescence
(XRF), the mineralogy using powder X-ray diffraction (PXRD) and thermogravimetry - differential
scanning calorimetry (TGA-DSC), the microtexture by environmental scanning electron
microscopy (ESEM) and the analysis of the porous system through mercury intrusion porosimetry
(MIP). The original mortars from the Castle were found to be earth and hydraulic lime-based, with
a porosity of between 21 and 38%. The Island building's mortars were mostly natural cementbased
and hydraulic lime-based, highlighting possible previous restoration works using natural
cement. These mortars portrayed a porosity lower than mortars from the Castle (18 to 30%). The
raw materials used on these monuments include feldspar aggregates, possibly from the West
Coast (Cape Town) and sub-hydraulic lime.
Phase 2 of this study involved the designand development of eight different mortar mixes following
a unique procedure invented in this research work. The freshly mixed mortars were evaluated for
consistency. In contrast, the hardened cubes of 40 40 40 mm and beams of 40 40 160
mm were evaluated for compatibility and durability using destructive (hygric tests, ageing tests
through salt crystallisation and freeze-thaw cycles, compressive and centre point loading flexural
strength tests) and a non-destructive techniques (ultrasound pulse velocity, UPV). The durability
tests aimed to assess the new mortars' performance and verify their long-term existence in
restoration practice. The old mortars properties were also assessed against the new ones, and
restoration interventions made.
For restoration purposes, a hydrated lime-based mortar with a binder-to-aggregate ratio of 1:3 by
weight, made of West Coast Sea sand and 5% seashell content with a porosity of 24% proved to
be the most durable among the eight produced mortars. Meanwhile, the aggregates of similar
sources with the addition of natural cement are proposed for earth mortars. The aesthetics of all
the mortars were difficult to achieve, given the ageing factor of the original materials. Thus, the
use of colour-enhancing pigments is recommended. A standardised guideline for producing
compatible and durable mortars has been documented for restorers to execute the works on
historic structures properly. This research confirmed that the compatibility and durability of
heritage mortars depends not only on their performance, but also on their ability to match the
properties of the existing materials with appropriate application techniques by skilled masons.
existence ensures generational knowledge of past events, milestones, construction
developments and evolution in materials, designs, concepts, and practices throughout the
centuries. It is indisputable that heritage buildings’ survival against human behaviour, natural
disasters, and environmental and atmospheric attacks through the years has proven the use of
durable materials for their construction. Inevitable decay due to long-term exposure to
deterioration factors is a common problem for these monuments. This has resulted in a need to
execute restoration works to reinstate the heritage structures to their original appearance,
physical state, and strength for extended survival.
Several restoration projects in the past and recent times have been performed to reinstate the
missing elements on heritage structures, masonry mortars in particular. Unfortunately, many of
these restoration interventions on these structures have been erroneous and led to poor results
from a restoration standpoint, especially regarding aesthetic criteria. The most common mistake
in restoration work is the use of incompatible restoration materials due to a misunderstanding of
the materials used on these historic wonders. This has resulted in a waste of resources due to
consequential repeated restoration projects and the loss of original structural concepts in terms
of appearance and integrity. These common mistakes can potentially threaten the historical
significance of heritage structures. The problem extends further to designing and producing
mortars that are compatible and their assessment for compatibility and durability.
To overcome the problems associated with material incompatibility, researchers and heritage
restorers have discovered the applicability of analysing original mortars for their chemical,
mineralogical, physical, and mechanical properties preceding the execution of restoration works
(ICOMOS, Venice Charter, 1964). The experimental analysis of these mortars is believed to offer
promising results for the long-term survival of restored historical masonries. To align the current
study with the literature regarding sustainable and compatible restoration of historical mortars,
two phases were examined. The initial phase of this study comprised an experimental analysis of
the samples extracted from the masonry joints, the floor, plaster, and renders of the ancient
colonial edifice in Cape Town, South Africa, Castle of Good Hope built in 1666. The building
precincts on Robben Island were constructed between 1700s and 1800s. A total of nine
representative samples were carefully extracted from the Castle of Good Hope, twelve from the
Pre-primary school building and three samples from the Maximum-security prison on Robben
Island for analysis. The sample size was decided based on material availability, considering the
restriction associated with causing as little destruction as possible on historical structures. Phase 1 of this doctoral thesis involved the determination of the aesthetic properties of the
collected mortars using spectrophotometry, the chemical composition through X-ray fluorescence
(XRF), the mineralogy using powder X-ray diffraction (PXRD) and thermogravimetry - differential
scanning calorimetry (TGA-DSC), the microtexture by environmental scanning electron
microscopy (ESEM) and the analysis of the porous system through mercury intrusion porosimetry
(MIP). The original mortars from the Castle were found to be earth and hydraulic lime-based, with
a porosity of between 21 and 38%. The Island building's mortars were mostly natural cementbased
and hydraulic lime-based, highlighting possible previous restoration works using natural
cement. These mortars portrayed a porosity lower than mortars from the Castle (18 to 30%). The
raw materials used on these monuments include feldspar aggregates, possibly from the West
Coast (Cape Town) and sub-hydraulic lime.
Phase 2 of this study involved the designand development of eight different mortar mixes following
a unique procedure invented in this research work. The freshly mixed mortars were evaluated for
consistency. In contrast, the hardened cubes of 40 40 40 mm and beams of 40 40 160
mm were evaluated for compatibility and durability using destructive (hygric tests, ageing tests
through salt crystallisation and freeze-thaw cycles, compressive and centre point loading flexural
strength tests) and a non-destructive techniques (ultrasound pulse velocity, UPV). The durability
tests aimed to assess the new mortars' performance and verify their long-term existence in
restoration practice. The old mortars properties were also assessed against the new ones, and
restoration interventions made.
For restoration purposes, a hydrated lime-based mortar with a binder-to-aggregate ratio of 1:3 by
weight, made of West Coast Sea sand and 5% seashell content with a porosity of 24% proved to
be the most durable among the eight produced mortars. Meanwhile, the aggregates of similar
sources with the addition of natural cement are proposed for earth mortars. The aesthetics of all
the mortars were difficult to achieve, given the ageing factor of the original materials. Thus, the
use of colour-enhancing pigments is recommended. A standardised guideline for producing
compatible and durable mortars has been documented for restorers to execute the works on
historic structures properly. This research confirmed that the compatibility and durability of
heritage mortars depends not only on their performance, but also on their ability to match the
properties of the existing materials with appropriate application techniques by skilled masons.
Additional information
Thesis (DEng (Civil Engineering))--Cape Peninsula University of Technology, 2023
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